Studies in tlic Calcitc Group. 217 



contains some twenty-nine measurements that are given with ^ 

 analyses of the mineral. The average of these angles is 73° 45'. 

 The average variation from this angle is less than 4' while the 

 greatest difference recorded was 2^,'. With the other members 

 of the group the number of recorded angles of analyzed material 

 is much smaller. The writer has been able to supplement these 

 in some case by measurements on material previously analyzed. 

 There are seven available measurements of the cleavage angle 

 on calcite which yield an average of 74° 56'. The average varia- 

 tion from this angle is 15'. The greatest variation is equal to 

 ^2' and this happens in the case 'of a specimen that does show 

 considerable amounts of other molecules present. The signifi- 

 cance of this, if there is any, is not readily seen however. The 

 variations in the angles are larger than perhaps would be 

 expected but they are irregular in character. If the variation was 

 dtie to a definite cause, such as a change in composition, it ought 

 to be in one direction only and, presumably in the present instance, 

 in the direction of the smaller angles, since the cleavage angle of 

 calcite is the largest of the group. In the case of rhodochrosite 

 the average of four angles was 72° 53', the average variation was 

 16' and the greatest 23'. With siderite only five angles were avail- 

 able. Their average was 73° 3', the average variation 8' and the 

 maximum difference 14'. \\^ith magnesite the average of six 

 measurements was ^2° 42'. The average departure from this 

 angle w^as 9' and the greatest 28'. 



The above data are not as extensive as could be desired nor as 

 carefully controlled. Still, the recorded variations in the cleav- 

 age angles are in most cases within the limits of natural errors 

 of observation. While perhaps it cannot be positively stated that 

 no variation in crystal angles takes place in the members of the 

 Calcite Group with changes in composition it is certain that any 

 such variation must be small. For the purposes of the present 

 study any possible small variation must be disregarded and the 

 angles considered as constant in their values. In other words, 

 then, the cr^^stal lattice of calcite, and likewise that of the other 

 members of the group, remains the same as far as its arrange- 

 ment and angles are concerned in spite of any isomorphous 

 replacement that may occur. The only other variation possible 

 would be in the spacing of the molecular layers which would of 

 course still involve a change in the volume of the unit cells. 



